348 SIE A. GEIKIE ON THE TEETIAET [May 1896,. 



Continuing still eastward, we find the feebly stratified tuff (a) to 

 be perhaps 200 feet thick. It forms a grassy declivity that descends 

 from the basalt-escarpment above to the grass-covered platform 

 which overlies a lower group of basalts. The visible portion of 

 this tuff presents a thoroughly volcanic character, being made up of 

 the usual dull dirty-green granular paste, through which are 

 dispersed angular and rough lumps of slag and pieces of more solid 

 basalt varying up to 1 or 2 feet in length. These stones are 

 generally disposed parallel to the indistinct bedding, but are some- 

 times placed on end as if they had assumed that position on falling 

 from an explosive shower. Among the smaller stones, pieces of a 

 finely vesicular basic pumice are frequent and are among the most 

 strikingly volcanic products of the deposit. From a characteristic 

 sample of these stones, a thin slice was prepared and placed in 

 Mr. Harker's hands. The following are his observations on it : — 

 'Avery compact dark-grey rock, amygdaloidal on a minute scale. 

 The lighter grey crust is probably due merely to weathering, and 

 the specimen seems to be a distinct fragment, not a true bomb. 



' The slice [6662] shows it to be essentially a brown glass with 

 only occasional microscopic crystals of a basic plagioclase. It has 

 been highly vesicular, and the vesicles are now filled by various 

 secondary products, including a chloritic mineral, nearly colourless 

 and singly refracting in thin section, and a zeolite.' 



Tracing now the tuff from the western side of the vent, we can 

 follow it to a greater distance. No abrupt line can be detected 

 here, any more than on the other side between the agglomerate and 

 the tuff. The latter rock extends under the overlying plateau of 

 basalt, at least as far west as Portree Loch, a distance of fully a 

 mile, but rapidly diminishes in thickness in that direction. Traces 

 of what is probably the same tuff can be detected between the 

 basalts at Ach na Hannait, more than 3 miles to the south. It 

 is thus probable that from the Portree vent fragmentary discharges 

 took place over an area of several square miles. 



Above the agglomerate of this vent two lavas may be seen to 

 start towards opposite directions. One of these (c in fig. 11) 

 begins immediately to the east of the two dykes. It is a dull 

 prismatic basalt with a slaggy bottom, its vesicles being pulled out 

 in the direction of the general bedding of the section. It descends 

 by a twist or step, and then lies on the gently inclined surface of 

 the tuff which dips towards the agglomerate. Farther east it 

 increases in thickness and forms the lowest of the basalt-sheets of 

 the cliff. The lava that commences on the western side of the vent 

 (d in fig. 11) is a massive jointed basalt, which, though not seen at 

 the vent, appears immediately to the west of it and rapidly swells 

 out so as to become one of the thickest sheets of the locality. It 

 lies upon the rudely bedded tuff, and is covered by the other basalts 

 of the cliff. 



That these two basalts came out of this vent cannot be affirmed. 

 If they did so at different times, their emission must have been 

 followed by the eruption which cleared the funnel and left the 



